The present invention relates generally to metering or controlling the flow rate of a descending molten metal stream and more particularly to the electromagnetic metering of such a stream.
Descending molten metal streams are employed in metallurgical processes such as the continuous casting of steel. In continuous casting, a stream of molten metal descends from an upper container, such as a ladle or a tundish, into a lower casting mold. The rate of flow of the descending molten metal stream has been conventionally controlled or metered by refractory mechanical devices such as refractory metering nozzles, refractory stopper rods or refractory sliding gates. All of these mechanical devices have a tendency to plug when refractory particles, suspended in the molten metal at a location upstream of the metering device, adhere to the refractory walls of the metering device, reducing the flow of the molten metal through the metering device.
Electromagnetic forces have been used in known metering systems to control the flow of a descending stream of molten metal in order to minimize or eliminate the above-described problems which arise when employing mechanical metering devices. In such systems, the stream of molten metal is surrounded by a primary coaxial coil of electrically conductive material, and an alternating electric current is flowed through the primary coil which generates a magnetic field which in turn induces eddy currents in the descending stream of molten metal. The net result of all of this is the production of a magnetic pressure which pinches or constricts the molten metal stream, reducing its cross-sectional area either at the coil or therebelow, depending upon whether the magnetic pressure is greater or less than the pressure head due to the stream.
More particularly, when the magnetic pressure is less than the pressure head due to the stream, the velocity of the descending stream, within the region of the magnetic field (hereinafter referred to as an upstream portion of the stream), is reduced by the magnetic pressure; however, the cross-sectional area of the stream is not reduced at its upstream portion. At that portion of the descending stream which is downstream of the magnetic field (hereinafter referred to as the downstream portion of the stream), there is no substantial magnetic pressure, the velocity of the downstream portion increases, and the stream there undergoes a constriction in its cross-sectional area to maintain a volume flow rate in the downstream portion equal to the volume flow rate in the upstream portion.
If the magnetic pressure exceeds the pressure due to the stream head, the stream will undergo a constriction in cross-sectional area in the region of the magnetic field (the stream's upstream portion). This is because so-called rotational flow occurs in the region of the magnetic field when the magnetic pressure exceeds the pressure head due to the stream. More particularly, stream flow in the center of the stream is in an upstream direction, while stream flow at the periphery of the stream is in a down stream direction; and the net flow in a downstream direction will appear as a constriction in the stream's cross-sectional area beginning in the region of the magnetic field (the stream's upstream portion).
It is desirable to operate the electromagnetic metering system under conditions of optimum electromagnetic efficiency. That efficiency is optimized when the magnetic pressure is relatively high and the power loss in the system is relatively low. Power losses occur in the primary coil which surrounds the descending stream of molten metal and in the stream of molten metal itself. Power losses are manifest as heat in both the primary coil and in the molten metal stream. Power loss in the primary coil is the limiting factor in determining the maximum available current and the generated magnetic field. Also, power loss in the molten metal may raise the temperature of the molten metal stream beyond tolerable limits.
The heat in the coil resulting from power loss there can be dissipated by cooling the coil with a circulating cooling fluid, but, as a practical matter, there is a limit to the amount of heat which can be carried away from the coil by cooling fluid. Overheating of the coil due to excessive power loss is intolerable.